Carrageenans comprise a class of polymeric carbohydrates which are obtainable by extraction of certain species of the class Rhodophyceae (red seaweed). In an idealised carrageenan the polymeric chain is made up of alternating A- and B-monomers thus forming repeating dimeric units. However in crude seaweed and thus in processed and purified carrageenans as well this regularity is often broken by some monomeric moieties having a modified structure.
Some carrageenans present particularly desirable hydrocolloid characteristics in the presence of certain cations and thus exhibit useful properties in a wide range of applications. Accordingly carrageenans are used as gelling and viscosity modifying agents in food as well as in non-food products, such as dairy products, gummy candy, jams and marmalade, pet foods, creams, lotions, air fresheners, gels, paints, cosmetics, dentifrices etc.
In the aforementioned applications the carrageenans are used either as a refined carrageenan (RC) product or as a semirefined carrageenan (SRC) product containing other seaweed residues.
As stated above the carrageenans comprise alternating A- and B-monomers. More specifically the carrageenans comprise chains of alternating moieties of a more or less modified D-galactopyranose in a α(1→3) linkage and a more or less modified D-galactopyranose in a β(1→4) linkage, respectively. The different types of carrageenans are classified according to their idealised structure as outlined in Table 1 below.
TABLE 1Carrageenan3-linked residue (= B)4-linked residue (= A)Beta (β)Beta-D-galactopyranose3,6-anhydrogalactopyranoseKappa (κ)Beta-D-galactopyranose-3,6-anhydrogalactopyranose4-sulphateIota (ι)Beta-D-galactopyranose-3,6-anhydrogalactopyranose-4-sulphate2-sulphateMu (μ)Beta-D-galactopyranose-galactopyranose-4-sulphate6-sulphateNu (ν)Beta-D-galactopyranose-galactopyranose-2,6-4-sulphatedisulphateLambda (λ)Beta-D-galactopyranose-galactopyranose-2,6-2-sulphate (70%) anddisulphategalactopyranose (30%)(for Chondrus)Theta (θ)Galactopyranose-2-sulphate3,6-anhydrogalactopyranose-(70%) and galactopyranose2-sulphate(30%) (for Chondrus)Xi (ξ)Beta-D-galactopyranose2-Alpha-D-galactopyranose-2-sulphate
Normally the polymer chains originating from seaweed deviate from the ideal structure in having irregularities present, such as eg. single moieties within the chain possessing a higher or lower number of sulphate groups. Also co-polymer types (or hybrid types) of carrageenans having two alternating sequences each representing different repeating dimeric units of two monomers are present in some seaweed species. Accordingly a vast array of different carrageenan materials having different properties exists.
The extent of gelling ability of the different types of carrageenans is inter alia determined by the amount of hydrophilic groups in the galactopyranose rings, molecular weight, temperature, pH and type and concentrations of salts in the solvent with which the hydrocolloid is mixed.
For gelling purposes, organoleptic and water binding purposes as well as texture and viscosity modifying purposes the most interesting and widely used carrageenans are the kappa-, iota-, theta- and lambda-carrageenans. These are not all present in the crude seaweed, but some of these are obtained by alkali modification of precursor carrageenans (mu-, nu- and lambda-carrageenan respectively) present in the crude seaweed according to the following reaction scheme:μ-carrageenan+OH−→κ-carrageenanν-carrageenan+OH−→ι-carrageenanλ-carrageenan+OH−→θ-carrageenan
Thus by alkali treatment of crude seaweed an intramolecular ether bond is formed within one of the ring moieties in the dimeric units of the carrageenan polymer providing less hydrophilic character to the polymer and accordingly rendering the polymer a more powerful gelling agent. The gelling properties are caused by the carrageenans organizing in a tertiary helical structure.
The kappa and iota structures (and their precursors) differ only by one sulphate group and are in fact always to some extent found in the same molecular chains from one seaweed material, and for this reason this group of carrageenan structures are called the “kappa family” of carrageenan structures. Almost pure kappa/mu respectively iota/nu providing seaweed exist, however, as do seaweeds that provide more equally balanced copolymers or “kappa/iota hybrids”.
Likewise, the xi and lambda (and its modified structure, theta after processing) are always found in distinct seaweed material which gives rise to the term “lambda family” for this group of carrageenan structures.
Whereas the isolated lambda- and theta-carrageenans are water soluble under almost every condition of temperature and salt concentration, the kappa- and iota-carrageenans—in the potassium salt form—are insoluble in cold water. All of the above carrageenans are soluble in hot water. The kappa- and iota-carrageenans are able to form gels in the presence of K+, Ca2+, Mg2+, Ba2+, Sr2+ and NH4+. The lambda and theta-carrageenans on the other hand do not form gels.
Some commercially available red seaweed species or populations contain only one carrageenan type (and its precursor). These are called “mono component seaweeds” in the present application. The commercially available seaweed Eucheuma cottonii (also known in the scientific literature as Kappaphycus alvarezii (Doty)) belongs to this category containing only one family of carrageenans, the “kappa family”.
Other examples of commercially available mono-component seaweed are Eucheuma spinosum (also known in the scientific literature as Eucheuma denticulatum), Hypneaspp. and Furcellaria spp.
However many available red seaweed species or populations contain at least two carrageenan types (including some of their precursors). These are in the present application called “bi-component seaweeds”. The commercially available seaweed Chondrus crispus belongs to this category, containing the “kappa family” as well as the “lambda family” of carrageenan structure, reportedly it maybe in the ratio of 70% kappa and 30% lambda. Other examples of commercially available bi-component seaweeds are several species from the Gigartina genus.
In the present application the term “gelling carrageenan” will be used for those carrageenan types which are able to form gels. Thus, the kappa family of carrageenans are “gelling carrageenans”, whereas the lambda family of carrageenans are not. The term “gelling carrageenan precursor” denotes in the present application a carrageenan precursor which becomes gelling after alkali modification. Thus the precursor itself may be non-gelling.
Traditionally carrageenans have been manufactured by extraction processes. Thus after wash, the seaweed has been subjected to an extraction with water at high temperature. The liquid extract is then purified by centrifugation and/or filtration. After this, the hydrocolloid is obtained either by evaporation of water or by selective precipitation by a potassium salt, or by an alcohol, such as isopropanol. This method of manufacture yields a pure and concentrated product, but suffers from high production cost.
U.S. Pat. No. 2,811,451 (Tjoa) discloses a treatment of seaweed wherein the seaweed is first rinsed and crushed and extracted with water (neutral, acidic or alkaline). By extraction at different temperatures, hydrocolloids with different properties are obtained. The obtained extracts may be used as is or may be further processed to obtain a powdery hydrocolloid.
In U.S. Pat. No. 3,094,517 (Stanley) a typical homogenous process for making carrageenan is disclosed. The process involves the use of an alkali, preferably calcium hydroxide. An excess of calcium hydroxide, which may amount to 40% to 115% of the weight of carrageenan present in the seaweed, has proven especially effective. The mixture of seaweed and alkali is then heated to temperatures ranging from 80° C. to 150° C., for a period of 3-6 hours. The excess alkali may be recovered for reuse, after which filter aid is added and filtration accomplished by any suitable type of equipment, while the mixture is still hot. The filtered extract is then neutralized using any suitable acid. When filtered, the extract is drum dried, spray dried or coagulated with alcohol. When alcohol precipitation is employed, the resulting coagulate is dried using conventional methods.
Rideout et al. in U.S. Pat. No. 5,801,240 refer to a prior art method for the production of semi refined or crude carrageenan, and U.S. Pat. No. 5,801,240 relates to improvements to this process. The method of Rideout et al. involves a number of steps: First the raw seaweed is cleaned and sorted. The cleaned and sorted seaweed is then rinsed at ambient temperature with either fresh water or a recycled potassium hydroxide wash. The seaweed is then placed in an aqueous potassium hydroxide cooking solution at 60-80° C. (2 hours at 12 wt % KOH or 3 hours at 8 wt % KOH) to modify the carrageenan and to dissolve some of the alkali soluble sugars. After cooking, the seaweed is removed and drained, and is then put through a series of wash steps to reduce the pH, to wash away residual potassium hydroxide, and to dissolve sugars and salts. Lastly, the resulting semi refined carrageenan is chopped, dried and ground. The inventive process by Rideout et al. further comprises the steps of monitoring the reaction progress by measuring the oxidation-reduction potential and stopping the reaction when an equilibrium as measured by a predetermined constant value of this potential is reached.
Thus, according to prior art, at least some carrageenans may be manufactured by a homogeneous process in which the carrageenan enters into solution, as well as by a heterogenous process, wherein the carrageenan remains undissolved. In the carrageenan manufacturing industry, however, the heterogenous process is preferred as this process does not require huge amounts of water for handling the very viscous extracts of carrageenan obtained when the carrageenan dissolves.
Accordingly, the most widely used way of conducting a heterogenous process according to the prior art is by reacting wet seaweed with KOH in a hot solution, as KOH quite efficiently provides for alkali modification of precursor carrageenans as well as for suppressing the solubility of the modified carrageenan, thus enabling low reaction volumes.
Thus, as both K+ and OH− are needed from the added KOH, it is not possible to cost-optimize the concentration of each of these individually without having to add the needed ion in the form of another salt, meaning an extra cost.
Although the seaweed is dried prior to transportation due to transportation cost concerns, the seaweed is traditionally supplied in a wet state in the alkali modification step in the processes for the manufacturing of SRC and RC according to the methods of the prior art by washing the seaweed. The reason for this washing prior to the alkali modification step is that it has previously been considered advantageous to wash out any residual salts from the seaweed. Also residues of sand and other contaminating matter has been washed out this way.
One drawback of the prior art method using KOH as the alkali is that KOH is an expensive chemical. Thus according to the prior art method of Rideout et al., supra, the KOH present in the seaweed is simply washed out via a series of subsequent washing steps to obtain a semirefined carrageenan product. Also, in a preferred embodiment of this prior art method of Rideout et al., the step following the alkali processing step is an acidic neutralization step using e.g. aqueous HCl. In this way of conducting the washing, the recovery of the alkali is excluded. Another consequence of this non-recovery of alkali, is that considerable amounts of anthropogenic chemicals will end up in the effluent and thus very likely also in the environment.
Accordingly it would be desirable to provide a method for the manufacture of carrageenans in a heterogenous process having a process set-up which enables a high degree of recovery of the alkali employed for the modification of precursor carrageenan in the reaction.
It has now been found, that a special process set-up for the manufacture of carrageenans comprising a heterogenous reaction in which seaweed, in particular dry seaweed, is reacted in an aqueous alkaline medium, a recovery process comprising one or more lye recovery sections, optionally washing and further conventional work-up, enables a very high degree of recovery of the aqueous alkaline medium remaining after the reaction, thus providing for substantial savings in consumption of alkali employed as well as savings in costs for neutralizing the effluent due to the reduction of the amount of alkali in the effluent.